1. Field of the Invention
This invention pertains to noval powder coating compositions which may be electrostatically applied to electroconductive substrates and subsequently thermally cured to give hard durable finishes.
2. Description of the Prior Art
There are several techniques for coating substrates with protective and/or decorative resinous materials. The three best known ways, however, are (1) applying the resin as a solution or emulsion thereof and evaporating the solvent (e.g., latex paints), (2) electrodeposition, and (3) electrostatic powder coating. The powder coating technique has advantages over the other two in that this technique makes more efficient use of the resinous coating materials than does the conventional spray coating of solvent-based systems and there are no solvent fumes being released, and the machinery used in the powder coating process is relatively inexpensive when compared to the capital outlay necessary for electrodeposition apparatus. These are substantial advantages which perhaps explain the continuing search by industry for new and better powder coatings.
Wingler et al. published a review article entitled, "Thermosetting Coatings Based on Acrylate-Styrene Copolymers for the Electrostatic Powder Spray Application" in Farbe Und Lack/Nr. 11, 1972. This article summarizes many of the techniques used in the electrostatic powder spray process.
Labana et al. describe various powder coating compositions based on ethylenically unsaturated glycidyl esters and dicarboxylic acids and flow control agents in U.S. Pat. Nos. 3,752,870 and 3,758,632 through 3,758,634.
Hartman et al. presented a technical review in Ind. Eng. Chem. Prod. Res. Develop. Vol. 12, No. 3, 166 (1973) on ambient curing polymer coatings. On page 1967 of this article, the authors described enamels based on oxazoline drying oils-acrylic copolymers which are alleged to be especially useful as automotive refinish enamels.
Arlt et al. described a powder coating composition in U.S. Pat. No. 3,752,793 which consisted of a mixture of (1)(a) a copolymer of styrene and certain substituted styrenes, acrylonitrile, methacrylonitrile or mixtures thereof, (b) acrylic acid esters, (c) methyl methacrylate, (d) acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid or mixtures thereof; which copolymer had an average molecular weight of from 3,000 to 20,000, and (2) a bis or trisoxazoline of a certain defined structure and (3) auxiliary agents such as pigments and flowing agents. Arlt et al. described a similar powder coating composition in German Offenlegungsschrift No. 2,155,257 wherein he replaced the bis and trisoxazolines in the aforementioned U.S. patent with certain bis and tris(5,6-dihydro-4H-1,3-oxazines). The Arlt et al. references represent the closest known art to the invention hereafter described. The "cross-link" in the polymers there-described have an ester group(s) ##STR1## adjacent to the polymer backbone with internal amide ##STR2## groups, and will therefore be referred to as ester-amides. This arrangement is the inverse of the instant polymers which are amide-esters.